SW6210202_SWM NARRATIVE_20210215

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Anniversary
E N G I N E E R S
FT. BRAGG SOF ADDITION STORMWATER MANAGEMENT NARRATIVE
The purpose of this land disturbance is to construct a new military operations support facility.
The existing site consists of two buildings, a parking lot, and internal roadways totaling an approximate
impervious area of 3.0 acres. The proposed development will include the demolition of existing
structures and the construction of a 30,000 SF -footprint building, approximately 277 parking stalls, and
internal roadways totaling an approximate impervious area of 4.5 acres. Six bioretention cells will be
constructed to retain additional runoff resulting from the 951h percentile rainfall event, post -developed
conditions. Four of the six proposed bioretention cells will discharge to an existing detention area, and
the two remaining bioretention cells will discharge to an appropriately-riprapped headwall.
The bioretention cells were designed to retain the additional runoff generated by the 951h
percentile rainfall event, which exceeds the state -required retention of the first 1". Most post -
developed runoff will result from the construction of the proposed building and its associated parking
stalls and internal roadways. According to the USDA, as well as a geotechnical report released by the
USACE in August of 2019, in situ soils are of Hydrologic Soil Group "A". However, due to the unknown
nature of fill soil quality and compaction, all bioretention cells were designed to include underdrain
systems. In addition to accommodating for unknown fill quality, the underdrains are also likely to reduce
bioretention cell maintenance burden when compared to similar bioretention cells without underdrain
systems. Each bioretention cell underdrain design calls for four (4) cleanouts to be placed no more than
10' from adjacent cleanouts.
Small portions of the post -developed site stormwater runoff will not be captured due to
topographical constraints. However, the overall condition of the site, regarding stormwater runoff
quality and quantity, will be improved. The post -developed conditions' peak discharge rates and
hydraulic volumes for all rainfall events will be less than those of the pre -developed conditions.
990 Hammond Drive, Suite 900, Atlanta, Georgia 30328
Phone: [phone] Fax: [phone] loweengineers.com